WG1 - Summary for Policymakers

Figure 1: Variations of the Earth's surface temperature over
the last 140 years and the last millennium.

(a) The Earth's surface temperature is shown year by year (red bars)
and approximately decade by decade (black line, a filtered annual curve
suppressing fluctuations below near decadal
time-scales). There are uncertainties in the annual data (thin black whisker
bars represent the 95% confidence range) due to data gaps, random instrumental
errors and uncertainties, uncertainties in bias corrections in the ocean
surface temperature data and also in adjustments for urbanisation over the
land. Over both the last 140 years and 100 years, the best estimate is that
the global average surface temperature has increased by 0.6 ± 0.2°C.

(b) Additionally, the year by year (blue curve) and 50 year average (black
curve) variations of the average surface temperature of the Northern Hemisphere
for the past 1000 years have been reconstructed from "proxy" data
calibrated against thermometer data (see list of the main proxy data in
the diagram). The 95% confidence range in the annual data is represented
by the grey region. These uncertainties increase in more distant times and
are always much larger than in the instrumental record due to the use of
relatively sparse proxy data. Nevertheless the rate and duration of warming
of the 20th century has been much greater than in any of the previous nine
centuries. Similarly, it is likely7 that the 1990s have been the warmest
decade and 1998 the warmest year of the millennium.
[Based upon (a) Chapter 2, Figure
2.7c and (b) Chapter 2, Figure
2.20]

The Third Assessment Report of Working Group I of the
Intergovernmental Panel on Climate Change (IPCC) builds upon past assessments
and incorporates new results from the past five years of research on climate
change1.
Many hundreds of scientists2
from many countries participated in its preparation and review.

This Summary for Policymakers (SPM), which was approved by IPCC member governments
in Shanghai in January 20013,
describes the current state of understanding of the climate system and provides
estimates of its projected future evolution and their uncertainties. Further
details can be found in the underlying report, and the appended Source Information
provides cross references to the report's chapters.

An increasing body of observations gives a collective picture of a warming
world and other changes in the climate system.

Since the release of the Second Assessment Report (SAR4),
additional data from new studies of current and palaeoclimates, improved analysis
of data sets, more rigorous evaluation of their quality, and comparisons among
data from different sources have led to greater understanding of climate change.

The global average surface temperature has increased over the 20th century
by about 0.6°C.

The global average surface temperature (the average of near surface air
temperature over land, and sea surface temperature) has increased since 1861.
Over the 20th century the increase has been 0.6 ± 0.2°C5,
6
(Figure 1a). This value is about 0.15°C
larger than that estimated by the SAR for the period up to 1994, owing to
the relatively high temperatures of the additional years (1995 to 2000) and
improved methods of processing the data. These numbers take into account various
adjustments, including urban heat island effects. The record shows a great
deal of variability; for example, most of the warming occurred during the
20th century, during two periods, 1910 to 1945 and 1976 to 2000.

Globally, it is very likely7
that the 1990s was the warmest decade and 1998 the warmest year in the instrumental
record, since 1861 (see Figure 1a).

New analyses of proxy data for the Northern Hemisphere indicate that the
increase in temperature in the 20th century is likely7
to have been the largest of any century during the past 1,000 years. It is
also likely7
that, in the Northern Hemisphere, the 1990s was the warmest decade and 1998
the warmest year (Figure 1b). Because
less data are available, less is known about annual averages prior to 1,000
years before present and for conditions prevailing in most of the Southern
Hemisphere prior to 1861.

On average, between 1950 and 1993, night-time daily minimum air temperatures
over land increased by about 0.2°C per decade. This is about twice the
rate of increase in daytime daily maximum air temperatures (0.1°C per
decade). This has lengthened the freeze-free season in many mid- and high
latitude regions. The increase in sea surface temperature over this period
is about half that of the mean land surface air temperature.

Temperatures have risen during the past four decades in the lowest 8 kilometres
of the atmosphere.

Since the late 1950s (the period of adequate observations from weather balloons),
the overall global temperature increases in the lowest 8 kilometres of the
atmosphere and in surface temperature have been similar at 0.1°C per decade.

Since the start of the satellite record in 1979, both satellite and weather
balloon measurements show that the global average temperature of the lowest
8 kilometres of the atmosphere has changed by +0.05 ± 0.10°C per
decade, but the global average surface temperature has increased significantly
by +0.15 ± 0.05°C per decade. The difference in the warming rates
is statistically significant. This difference occurs primarily over the tropical
and sub-tropical regions.

The lowest 8 kilometres of the atmosphere and the surface are influenced
differently by factors such as stratospheric ozone depletion, atmospheric
aerosols, and the El Niño phenomenon. Hence, it is physically plausible
to expect that over a short time period (e.g., 20 years) there may be differences
in temperature trends. In addition, spatial sampling techniques can also explain
some of the differences in trends, but these differences are not fully resolved.

Snow cover and ice extent have decreased.

Satellite data show that there are very likely7
to have been decreases of about 10% in the extent of snow cover since the
late 1960s, and ground-based observations show that there is very likely7
to have been a reduction of about two weeks in the annual duration of lake
and river ice cover in the mid- and high latitudes of the Northern Hemisphere,
over the 20th century.

There has been a widespread retreat of mountain glaciers in non-polar regions
during the 20th century.

Northern Hemisphere spring and summer sea-ice extent has decreased by about
10 to 15% since the 1950s. It is likely7
that there has been about a 40% decline in Arctic sea-ice thickness during
late summer to early autumn in recent decades and a considerably slower decline
in winter sea-ice thickness.

Global average sea level has risen and ocean heat content has increased.

Tide gauge data show that global average sea level rose between 0.1 and
0.2 metres during the 20th century.

Global ocean heat content has increased since the late 1950s, the period
for which adequate observations of sub-surface ocean temperatures have been
available.